1. Field of the Invention
The present invention relates to an optical element for transmitting light therethrough and modulating the light and a photodetector equipped therewith.
2. Related Background Art
Known as photodetectors utilizing light absorption of quantum intersubband transitions are QWIP (quantum well type infrared optical sensor), QDIP (quantum dot infrared optical sensor), QCD (quantum cascade type optical sensor), and the like.
They utilize no energy bandgap transitions and thus have such merits as high degree of freedom in designing wavelength ranges, low dark current, and operability at room temperature.
Among these photodetectors, the QWIP and QCD are equipped with a semiconductor multilayer body having a periodic multilayer structure such as a quantum well structure or quantum cascade structure. This semiconductor multilayer body generates a current due to an electric field component in the stacking direction thereof only when light incident thereon has such an electric field component, and thus is not photosensitive to light having no electric field component in the stacking direction (planar waves incident thereon in the stacking direction thereof).
Therefore, in order for the QWIP or QCD to detect light, it is necessary for the light to be incident thereon such that a direction of vibration of an electric field of the light coincides with the stacking direction of the semiconductor multilayer body. When detecting a planar wave having a wavefront perpendicular to an advancing direction of light, for example, it is necessary for the light to be incident on the semiconductor multilayer body in a direction perpendicular to its stacking direction, which makes the photodetector cumbersome to use.
There has hence been known a photodetector which, for detecting light having no electric field component in the stacking direction of the semiconductor multilayer body, a thin gold film is disposed on a surface of the semiconductor multilayer body and periodically formed with holes each having a diameter not greater than the wavelength of the light (see W. Wu, et al., “Plasmonic enhanced quantum well infrared photodetector with high detectivity”, Appl. Phys. Lett., 96, 161107 (2010)). In this example, the light is modulated so as to attain an electric field component in the stacking direction of the semiconductor multilayer body under a surface plasmonic resonance effect on the thin gold film.
There has also been known a photodetector in which a light-transmitting layer is disposed on a surface of a semiconductor multilayer body, while a diffraction grating constituted by a pattern of irregularities and a reflective film covering the same are formed on the light-transmitting layer (see Japanese Patent Application Laid-Open No. 2000-156513). In this example, the light is modulated so as to attain an electric field component in the stacking direction of the semiconductor multilayer body under the effects of diffraction and reflection of the incident light by the diffraction grating and reflective film.